JP7552937B2 - Object detection device - Google Patents

Description

The present invention relates to an object detection device that detects objects by transmitting and receiving sound waves such as ultrasound.

Patent Document 1 discloses a technology aimed at achieving both improved object detection performance and suppression of unnecessary detection of objects in an object detection device that detects objects using an ultrasonic sensor. In Patent Document 1, the fact that the exploration wave transmitted from the wave transmitting sensor is received by the wave receiving sensor directly from the wave transmitting sensor, rather than as a reflected wave from the object, is referred to as "sideways jumping." In Patent Document 1, as a countermeasure against the risk of the wave receiving sensor erroneously detecting the presence of an object when in fact there is no object due to sideways jumping, reception at the wave receiving sensor is prohibited for a predetermined period of time after a command to start transmitting exploration waves is output to the wave transmitting sensor.

JP 2018-105703 A

The object of the present invention is to provide an object detection device that can reduce the influence of sound waves received directly without passing through the object when detecting an object by transmitting and receiving sound waves.

The object detection device of the present invention detects an object by transmitting and receiving sound waves. The object detection device includes a transmitter that transmits modulated waves having a predetermined frequency band to an object, a first receiver that receives the sound waves and generates a first received signal indicating the reception result, a second receiver that receives the sound waves at a position farther away from the transmitter than the first receiver and generates a second received signal indicating the reception result, an adder that adds the first received signal and the second received signal to generate a third received signal, and a control unit that causes the transmitter to transmit modulated waves and detects an object based on the third received signal. The first and second receivers are arranged so that when the first and second receivers receive the modulated waves from the transmitter, the fluctuation range of the third received signal corresponding to the modulated waves is equal to or less than the fluctuation range of the first received signal corresponding to the modulated waves.

According to the object detection device of the present invention, when detecting an object by transmitting and receiving sound waves, the influence of sound waves that are received directly without passing through the object can be reduced.

FIG. 1 is a diagram for explaining an overview of an object detection device according to a first embodiment. FIG. 1 is a block diagram showing a configuration of an object detection device according to a first embodiment. FIG. 2 is a diagram showing an example of the arrangement of a transmitter and a receiver in the object detection device of the first embodiment; FIG. 1 is a diagram for explaining a transmission signal in an object detection device; FIG. 1 is a block diagram showing a functional configuration of a control unit in an object detection device according to a first embodiment. Graph for explaining an analytic signal in an object detection device Graph illustrating experimental results of the operation of the object detection device of embodiment 1 Graph showing experimental results when affected by direct waves FIG. 1 is a diagram showing an example of frequency characteristics of a receiver in an object detection device; FIG. 1 is a waveform diagram showing an example of a received wave signal when a direct wave in a first frequency band is received by an object detection device; FIG. 11 is a diagram showing the relationship between the shift time and the fluctuation width in the addition result of the received signal of FIG. 10 . FIG. 12 is a waveform diagram showing the received signal obtained by adding the received signal at the shift time with the smallest fluctuation range in FIG. FIG. 11 is a waveform diagram showing an example of a received wave signal when a direct wave in a second frequency band is received by an object detection device. FIG. 14 is a diagram showing the relationship between the shift time and the fluctuation width in the addition result of the received signal of FIG. 13 . FIG. 11 is a waveform diagram showing an example of a received wave signal when a direct wave in a third frequency band is received by an object detection device. FIG. 16 is a diagram showing the relationship between the shift time and the fluctuation width in the addition result of the received signal of FIG. 15 . FIG. 13 is a diagram showing another example of frequency characteristics of a receiver in an object detection device; FIG. 11 is a waveform diagram showing another example of a received wave signal when a direct wave in the first frequency band is received in the object detection device; FIG. 20 is a diagram showing the relationship between the shift time and the fluctuation width in the addition result of the received signal of FIG. 18 . FIG. 11 is a waveform diagram showing another example of a received wave signal when a direct wave of a second frequency band is received in an object detection device; FIG. 21 is a diagram showing the relationship between the shift time and the fluctuation width in the addition result of the received signal of FIG. 20 . FIG. 11 is a waveform diagram showing another example of a received wave signal when a direct wave of the third frequency band is received by the object detection device; FIG. 23 is a diagram showing the relationship between the shift time and the fluctuation width in the addition result of the received signal of FIG. 22 . FIG. 13 is a diagram for explaining an object detection device according to a second embodiment. FIG. 1 is a diagram showing a first modified example of the object detection device according to the first embodiment; FIG. 13 is a diagram showing a second modified example of the object detection device according to the first embodiment;

Below, an embodiment of the object detection device according to the present invention is described with reference to the attached drawings.

It goes without saying that each embodiment is an example, and partial substitution or combination of the configurations shown in different embodiments is possible. From embodiment 2 onwards, description of matters common to embodiment 1 will be omitted, and only the differences will be described. In particular, similar effects resulting from similar configurations will not be mentioned in each embodiment.

(Embodiment 1)
The configuration and operation of the object detection device according to the first embodiment will be described below.

1. Configuration 1-1. Overview An overview of the object detection device according to the first embodiment will be described with reference to FIG.

Figure 1 is a diagram for explaining an overview of an object detection device 1 of this embodiment. The object detection device 1 is a device that detects the distance to an object 3 by transmitting and receiving sound waves such as ultrasonic waves. Hereinafter, the direction in which the distance is detected between the object detection device 1 and the object 3 is defined as the Z direction, and the two directions that are perpendicular to the Z direction and perpendicular to each other are defined as the X and Y directions.

The object detection device 1 of this embodiment can be applied to various applications for detecting various objects 3. For example, when mounted on a moving body, the object detection device 1 can detect the distance at which the moving body approaches the surrounding objects 3, the presence or absence of obstacles, or the road surface condition. Alternatively, the object detection device 1 can be applied to applications such as vital sensing for measuring the heart rate or respiration of a living body by detecting minute displacements of various objects 3, or structural inspection of wiring, steps, etc. in various products.

The object detection device 1 emits sound waves, i.e. chirp waves W1, whose frequency is modulated according to each detection application, from the wave transmitter 10 toward the +Z side of the object 3, and receives the reflected wave, i.e. echo W2, of the chirp waves W1 from the object 3 with the wave receiver 11 to detect the object 3. At this time, the chirp waves W1 from the wave transmitter 10 generate direct waves W0 that reach the wave receiver 11 directly, without being reflected by the object 3, and the effect of the direct waves W0 on the detection accuracy of the object 3 becomes an issue.

As a countermeasure to the above problem, there is a method of setting a time period during which the reception of the direct wave W0 is prohibited (see Patent Document 1). However, in this conventional method, sound waves cannot be received during the time period during which the direct wave W0 is prohibited, so the echo W2 that arrives simultaneously with the direct wave W0 cannot be used for object detection. For example, the conventional method is difficult to apply to an object detection method that gives the chirp wave W1 a long chirp length. In addition, it is not possible to detect objects at close range according to the prohibited time period.

In contrast, in the present embodiment, the reception of the direct wave W0 is permitted rather than prohibited, and the object detection device 1 is configured to use multiple receivers 11, thereby reducing the effect of the direct wave W0 on the detection accuracy of the object 3, and further resolving the above problem. The configuration of the object detection device 1 is described below.

1-2. Device Configuration The configuration of the object detection device 1 of this embodiment will be described with reference to Fig. 2 and Fig. 3. Fig. 2 is a block diagram showing the configuration of the object detection device 1. Fig. 3 is a diagram showing an example of the arrangement of the wave transmitter 10 and the wave receiver 11 in the object detection device 1 of this embodiment.

2, the object detection device 1 of this embodiment includes a wave transmitter 10, two wave receivers 11a and 11b, a control unit 13, a memory unit 14, a wave transmitting circuit 15, and a wave receiving circuit 16. In the following, the receivers 11a and 11b are collectively referred to as the wave receiver 11.

Figure 3 shows an example of a configuration in which the object detection device 1 has a transmitter 10, a first receiver 11a, and a second receiver 11b arranged in order in the X direction. The transmitter 10 and receiver 11 are arranged, for example, on a substrate having a main surface along the XY plane. The distance d1 between the transmitter 10 and the first receiver 11a is greater than the distance d2 between the transmitter 10 and the second receiver 11b. Each distance d1, d2 can be specified within an appropriate allowable error range and can be measured as the distance between reference positions, for example, the center position of the transmitter 10 and the center position of each receiver 11.

In the object detection device 1 of this embodiment, the distance difference d0 between the distance d1 from the transmitter 10 to the first receiver 11a and the distance d2 from the transmitter 10 to the second receiver 11b is set from the viewpoint of reducing the influence of the direct wave W0 (FIG. 1). In other words, the first and second receivers 11a and 11b are positioned relative to the transmitter 10 according to the optimized distance difference d0. The optimization of the distance difference d0 in the object detection device 1 of this embodiment will be described later.

In the example configuration of the object detection device 1 shown in Figure 3, the two receivers 11a, 11b and the transmitter 10 are arranged on the same XY plane. This makes it easy to approximate the distance from the object 3 (Figure 1) located on the +Z side of the object detection device 1 to the first receiver 11a and the distance to the second receiver 11b, and to appropriately set the distance difference d0 from the transmitter 10 to each of the receivers 11a and 11b.

Returning to FIG. 2, the wave transmitter 10 is, for example, composed of a thermophone, which is a sound wave source that generates sound waves by heating air and then stopping the heating. With a thermophone wave transmitter 10, it is easy to widen the frequency of sound waves and perform various modulations, such as setting the frequency of sound waves according to the cycle of repeated heating and stopping of heating. Furthermore, by using a thermophone, the wave transmitter 10 can be configured to be small and lightweight. With a thermophone, for example, by applying pulse interval modulation to keep the pulse width short, it is easy to suppress heat generation and power consumption.

The transmitter 10 of this embodiment may be any of a variety of non-directional sound sources that do not have any particular directionality. The transmitter 10 may be a variable or fixed directional sound source. The transmitter 10 is not limited to a thermophone, and may be, for example, a piezoelectric resonant ultrasonic transducer.

The transmitter 10 generates ultrasonic waves having a frequency of, for example, 20 kHz or more as a modulated wave such as a chirp wave W1. The chirp wave W1 is modulated to have a predetermined frequency band by, for example, a down-chirp in which the frequency gradually decreases (for example, linearly) over a chirp length of a predetermined period.

The modulated wave of the transmitter 10 may use various modulation methods, not limited to the above, such as up-chirp or a spreading code such as an M-sequence code. Pulse width modulation may be used instead of pulse interval modulation. Furthermore, amplitude modulation may be performed instead of frequency modulation. The transmitter 10 may generate sound waves of various frequency bands, not limited to ultrasonic waves. The frequency band of the chirp wave W1 is set so that a frequency band twice the frequency band does not overlap with the frequency band.

The wave transmission circuit 15 is a drive circuit for the wave transmitter 10, and drives the wave transmitter 10 based on, for example, a wave transmission signal Sd input from the control unit 13. For example, when the wave transmitter 10 is a thermophone, the wave transmission circuit 15 can be configured using a switching transistor, a capacitor, an inductor, etc., and controls the on/off of the current flowing to the thermophone according to the wave transmission signal Sd. The wave transmission circuit 15 may set the frequency band, chirp length, intensity, directivity, etc. of the sound waves generated by the wave transmitter 10. Some or all of the functions of the wave transmission circuit 15 may be configured integrally with the wave transmitter 10 or the control unit 13.

The receiver 11 is composed of a microphone such as a MEMS (Micro Electro Mechanical System) microphone. The first and second receivers 11a and 11b are composed of, for example, the same type of product and have common frequency characteristics (see FIG. 9, etc.). The frequency characteristics of each receiver 11 may vary within an appropriate allowable error range.

Each of the receivers 11a and 11b receives sound waves from the outside and generates a reception signal Sr1 or Sr2 indicating the reception result. The receiver 11 is not limited to a MEMS microphone, and may be composed of other microphones having frequency characteristics capable of receiving wideband ultrasonic waves transmitted from the transmitter 10. For example, a condenser microphone may be used for the receiver 11. The receiver 11 may be omnidirectional or may have various directivities as appropriate.

The receiving circuit 16 constitutes, for example, an adder that adds together the multiple receiving signals Sr1, Sr2 from each receiver 11a, 11b. The receiving circuit 16 generates a receiving signal Sr that is the addition result of the multiple receiving signals Sr1, Sr2, and outputs the receiving signal Sr generated within a predetermined dynamic range to the control unit 13. The receiving circuit 16 may include various driving circuits for each receiver 11. Some or all of the functions of the receiving circuit 16 may be configured integrally with the receiver 11 or the control unit 13.

The control unit 13 controls the overall operation of the object detection device 1. The control unit 13 is composed of, for example, a microcomputer, and works with software to realize predetermined functions. The control unit 13 reads out data and programs stored in the memory unit 14 and performs various calculation processes to realize various functions. For example, the control unit 13 generates a transmission signal Sd for generating chirp waves in the transmitter 10 based on the data stored in the memory unit 14, and outputs it to the transmission circuit 15. The control unit 13 also has a calculation function for detecting an object 3 based on a reception signal Sr from the reception circuit 16 (described later).

The control unit 13 may be a hardware circuit such as a dedicated electronic circuit or a reconfigurable electronic circuit designed to realize a predetermined function. The control unit 13 may be configured with various semiconductor integrated circuits such as a CPU, an MPU, a DSP, an FPGA, and an ASIC. The control unit 13 may also be configured to include an analog/digital (A/D) converter and a digital/analog (D/A) converter.

The memory unit 14 is a storage medium that stores programs and data necessary to realize the functions of the control unit 13, and is composed of, for example, a flash memory. For example, the memory unit 14 stores data indicating the transmission signal Sd. An example of such data of the transmission signal Sd is shown in FIG. 4. In the example of FIG. 4, a pulse waveform of the transmission signal Sd for generating a down-chirp chirp wave W1 is shown.

1-3. Functional Configuration of the Control Unit The calculation function of the control unit 13 in the object detection device 1 of this embodiment will be described with reference to FIG.

Figure 5 is a block diagram showing the functional configuration of the control unit 13. The control unit 13 includes, for example, as functional units, fast Fourier transform (FFT) units 131a and 131b, a cross-spectral calculation unit 132, a Hilbert transform unit 133, inverse Fourier transform (IFFT) units 134a and 134b, and an analysis processing unit 135. Hereinafter, the FFT units 131a and 131b are collectively referred to as the FFT unit 131, and the IFFT units 134a and 134b are collectively referred to as the IFFT unit 134.

The control unit 13 inputs, for example, a transmission signal Sd from the memory unit 14 and a reception signal Sr from the reception circuit 16, and performs signal processing by each of the functional units 131 to 135. Each of the functional units 131 to 135 can operate periodically at a predetermined measurement frame rate (e.g., 30 frames/second) as described below.

Among the functional units 131 to 135, a series of processes from the FFT unit 131 to the IFFT unit 134 are performed to generate an analytic signal based on the transmitted wave signal Sd and the received wave signal Sr for each frame. The analytic signal is composed of the cross-correlation function of the transmitted wave signal Sd and the received wave signal Sr. The cross-correlation function is a function that indicates the correlation between the two signals Sd and Sr in the time domain.

The FFT unit 131a converts the transmitted wave signal Sd from the time domain to the frequency domain by calculating a fast Fourier transform on the transmitted wave signal Sd input to the control unit 13, and outputs the conversion result to the cross spectrum calculation unit 132. The FFT unit 131b performs a calculation similar to that of the FFT unit 131a on the received wave signal Sr input to the control unit 13, and outputs the conversion result to the cross spectrum calculation unit 132.

The cross spectrum calculation unit 132 calculates a cross spectrum from the results of the Fourier transform of each signal Sd, Sr by the FFT unit 131, and outputs the calculation results to the IFFT unit 134a and the Hilbert transform unit 133. The cross spectrum corresponds to the frequency components obtained by the Fourier transform of the cross-correlation function between the transmitted signal Sd and the received signal Sr.

The IFFT unit 134a performs an inverse fast Fourier transform on the input cross spectrum and outputs a signal I, which is the result of the conversion from the frequency domain back to the time domain, to the analysis processing unit 135. The signal I thus output represents the cross-correlation function between the transmitted and received signals Sd and Sr (hereinafter also referred to as the "in-phase component I").

The Hilbert transform unit 133 calculates the Hilbert transform of the input cross spectrum and outputs the transformation result in which each frequency component of the cross spectrum is shifted by π/2 to the IFFT unit 134b.

The IFFT unit 134b performs the same calculation as the IFFT unit 134a on the Hilbert transformed cross spectrum, and outputs the converted signal Q to the analysis processing unit 135. The signal Q thus output is in an orthogonal relationship with the in-phase component I (hereinafter also referred to as the "orthogonal component Q").

The analysis processing unit 135 generates an analytic signal having an in-phase component I as a real part and a quadrature component Q as an imaginary part, and performs processing on the analytic signal. The analytic signal thus generated based on the transmitted signal Sd and the received signal Sr represents an analytic function in the complex domain (see FIG. 6).

The various functions of the control unit 13 as described above may be realized, for example, by a program stored in the memory unit 14, or some or all of the various functions may be realized by a hardware circuit. In addition, in the control unit 13, the cross-correlation function may be calculated directly from the transmitted and received signals Sd and Sr by product-sum calculation, for example, instead of the process of performing an inverse Fourier transform after calculating the cross spectrum after the Fourier transform. For example, the control unit 13 may be equipped with a circuit such as an FPGA that performs product-sum calculations. In addition, the generation of the analytic signal in the control unit 13 is not limited to the Hilbert transform, and may be realized, for example, by a function of quadrature detection.

2. Operation The operation of the object detection device 1 configured as above will be described below.

In the object detection device 1 of this embodiment, the measurement operation for one frame is performed by causing the transmitter 10 to transmit a single chirp wave W1 corresponding to the data of the transmission signal Sd illustrated in Figure 5, and receiving the echo W2 of the chirp wave W1 with the receiver 11. The measurement operation for each frame is performed sequentially. For example, the control unit 13 of the object detection device 1 performs calculations on each of the functional units 131 to 135 for each measurement frame, and generates an analysis signal indicating the analysis result of the correlation between the transmission signal Sd and the reception signal Sr during the frame period.

Figure 6 is a graph for explaining the analytic signal z(t) in the object detection device 1. Figure 6 illustrates one frame's worth of analytic signal z(t). The analytic signal z(t) has a complex range due to the real part of the in-phase component I(t) that indicates the cross-correlation function between the transmitted signal Sd and the received signal Sr, and the imaginary part of the corresponding quadrature component Q(t).

The object detection device 1, for example, finds the envelope E(t)=|z(t)| of the analytic signal z(t) to detect the peak time _t0 . The peak time _t0 is the timing at which the amplitude |z(t)| of the analytic signal z(t) of one frame is maximum. In this way, by analyzing the timing corresponding to the reflection by the object 3 in the transmission and reception of the chirp wave W1 of the frame, it is possible to measure, for example, the propagation period until the transmitted chirp wave W1 is received as an echo W2 from the object 3.

By performing the above analysis, the object detection device 1 can accurately detect the distance to the object 3 from the propagation period of the echo W2 arriving from the object 3. Furthermore, the object detection device 1 may analyze the phase ∠z(t) in addition to the envelope E(t) in the analytic signal z(t) obtained by complexing the cross-correlation function, and may calculate the phase difference between successive frames, for example. This makes it possible to accurately detect, for example, minute displacements of the object 3.

2-1. Influence of direct waves Fig. 7 is a graph illustrating the experimental results of the object detection device 1 of this embodiment. In Fig. 7, a measurement experiment was conducted using a wall located 2.2 m away from the wave transmitter 10 and the wave receiver 11 of the object detection device 1 as the object 3 to be detected. The frequency band of the chirp wave W1 was set to a wide band from 40 kHz to 80 kHz.

Figure 7(A) illustrates a waveform diagram of the received signal Sr in the object detection device 1 of this embodiment. Figure 7(B) illustrates a waveform diagram of the analysis result of the received signal Sr in Figure 7(A), specifically showing the signal waveform of the envelope E(t) = |z(t)| of the analytic signal z(t) of the cross-correlation function.

The object detection device 1 of this embodiment transmits and receives chirp waves W1 modulated in a wide band, for example as described above, and analyzes the correlation between the transmitted signal Sd and the received signal Sr over the chirp length during a frame period, thereby achieving highly accurate object detection, such as measuring the distance to the object 3. In such an object detection operation, in order to receive the echo W2 from the object 3 for the chirp length, a relatively long time period during which the receiver 11 continues receiving is secured.

7(A) and (B), it is assumed that the reception range of the direct wave W0 and the echo range from the object 3 overlap each other. In other words, when performing the above-mentioned high-precision object detection operation, it is difficult to take measures against the influence of the direct wave W1, such as prohibiting reception of the direct wave W1 or excluding the reception range of the direct wave W0 from the analysis target in the received wave signal Sr.

7A illustrates an example of the receiving signal Sr corresponding to FIG. 7B in the object detection device 1 of this embodiment. Although the receiving signal Sr of this example contains a component of the direct wave W0, the amplitude of the receiving signal Sr falls within the dynamic range R1 of the receiving circuit 16 due to the arrangement of each receiver 11 of this embodiment.

In this case, even if there is overlap between the reception range of the direct wave W0 and the reception range of the echo W2 from the object 3, the peak time Pw0 of the direct wave W0 and the peak time Pw2 of the echo W2 from the object 3 can be obtained separately by cross-correlation analysis, as shown in Figure 7 (B). In this way, in the object detection device 1 of this embodiment, even if the direct wave W0 is received, the object 3 can be detected with high accuracy.

In contrast, Figure 8 illustrates a case in which accuracy is reduced due to the influence of the direct wave W0 in the object detection operation described above. Figure 8(A) illustrates a waveform diagram of the received signal Srx when there is the influence of the direct wave W0. Figure 8(B) illustrates a waveform diagram of the analysis result of the received signal Srx in Figure 8(A).

In the example of Figure 8 (A), clipping occurs in the received signal Srx due to the influence of the direct wave W0. In other words, the direct wave W0 causes a portion of the waveform amplitude to exceed the dynamic range R1 of the receiving circuit or receiver, and an accurate signal waveform of the received signal Srx cannot be obtained. In this case, as shown in Figure 8 (B), the noise level L1 in the cross-correlation analysis result is higher than in Figure 7 (B), and the deterioration of the signal-to-noise ratio makes it difficult to detect objects with high accuracy.

Therefore, in the object detection device 1 of this embodiment, the distance difference d0 for each receiver 11a, 11b is optimized to cancel the effect of the direct wave W0 to a degree that does not cause clipping in the receiver circuit 16 that adds the receiving signals Sr1, Sr2 from the first and second receivers 11a, 11b. This makes it possible to suppress the deterioration of the signal-to-noise ratio due to the effect of the direct wave W0 and reduce the effect of the direct wave W0 on object detection.

2-2. Optimization of Distance Difference Optimization of the distance difference d0 between the receivers 11a and 11b in the object detection device 1 of this embodiment will be described with reference to FIGS.

Figure 9 shows an example of the frequency characteristics of the receiver 11 in the object detection device 1. In this example, the frequency characteristics are illustrated when a Knowles microphone SPU0410LR5H is used as the receiver 11. The horizontal axis of Figure 9 indicates frequency, and the vertical axis indicates sensitivity.

In addition, FIG. 9 illustrates three frequency bands Fm, Fn, and Fw as examples of the frequency band of the chirp wave W1. The first frequency band Fm is 40 kHz or more and 80 kHz or less. The second frequency band Fn is 60 kHz or more and 80 kHz or less. The third frequency band Fw is 20 kHz or more and 80 kHz or less. According to the frequency characteristics in FIG. 9, for example, the sensitivity at a frequency of 60 kHz is lower than the sensitivity at a frequency of 80 kHz.

In the object detection device 1 of this embodiment, a wide band such as the first and second frequency bands Fm, Fn, and Fw is used for the chirp wave W1, so it is considered that the influence of the direct wave W0 cannot be offset against other frequency components even if only the phase difference of a single frequency is considered. For this reason, in this embodiment, as illustrated in FIG. 9, the distance difference d0 that can be offset between multiple receivers 11a and 11b is optimized while taking into account the frequency characteristics of the receiver 11 according to the frequency band of the chirp wave W1. In this case, the signal waveform when only the direct wave W0 of the chirp wave W1 is received by one receiver 11 is used.

Figure 10 shows an example of a received wave signal Sr1 when a direct wave W0 in the first frequency band Fm is received. The horizontal axis of Figure 10 is time, and the vertical axis is the signal level of the received wave signal Sr1. Figure 10 illustrates an example in which a direct wave W0 is received by the first receiver 11a having the frequency characteristics of Figure 9 when the chirp wave W1 is set to down-chirp in the first frequency band Fm.

In the example of Figure 10, amplitude fluctuations, or undulations, occur in the received signal Sr1. The cause of this undulation in the received signal Sr1 is due to the frequency characteristics of the first receiver 11a. For example, a flatter signal waveform can be obtained by using a microphone with a wider band and more stable frequency characteristics. Furthermore, the received signal Sr2 of the second receiver 11b has a waveform that is shifted from the waveform of the received signal Sr1 of the first receiver 11a by a time width corresponding to the distance difference d0 (hereinafter referred to as "shift time Δt").

In the optimization method of this embodiment, as shown in Figure 10, for example, attention is paid to the fluctuation width Vpp1 between the maximum signal level and the minimum signal level in the received signal Sr1 corresponding to the direct wave W0. Specifically, using the waveform of the received signal Sr1 as shown in Figure 10, a waveform obtained by adding two waveforms with a shift time Δt is calculated, and the fluctuation width Vpp in the waveform of the received signal Sr resulting from the addition is extracted. The calculation results when the shift time Δt is changed are shown in Figure 11.

Figure 11 shows the relationship between the shift time Δt and the fluctuation range Vpp in the received signal Sr resulting from the addition of the received signal Sr1 in Figure 10. For example, when Δt = 0, the fluctuation range Vpp level of the received signal Sr resulting from the addition is twice the fluctuation range Vpp1 of the received signal Sr1 before the addition. Figure 11 shows that the fluctuation range Vpp of the received signal Sr resulting from the addition changes approximately periodically with respect to the shift time Δt.

In the example of FIG. 11, in the periodic increase and decrease described above, multiple intervals T1 of the shift time Δt can be confirmed in which the fluctuation width Vpp after addition is equal to or less than the fluctuation width Vpp1 before addition. In this time interval T1, by using multiple receivers 11a and 11b, it is possible to reduce the influence of the direct wave W0 (i.e., the possibility of clipping) more than when using a single receiver 11. Therefore, in this embodiment, a shift time Δt included in such a time interval T1 is selected, and the distance difference d0 corresponding to the selected shift time Δt is determined as the optimization result. The distance difference d0 can be converted from the shift time Δt appropriately using the sound speed.

Figure 12 shows the received wave signal Sr resulting from addition at a shift time Δt = 9 μs, when the fluctuation width Vpp after addition in Figure 11 reaches its minimum peak P1. The distance difference d0 corresponding to this shift time Δt minimizes the influence of the direct wave W0 in the received wave signal Sr after addition. Furthermore, since the distance difference d0 is minimized, the object detection device 1 can be made more compact. Furthermore, the phase shift in the range of the detection direction of the object 3 can be minimized, and the directivity of the detection direction can be expanded.

Note that the object detection device 1 does not have to use a distance difference d0 that corresponds strictly to the shift time Δt in FIG. 11. The above effect can be achieved even if the distance difference d0 corresponds within an appropriate range of allowable error. For example, in the section of distance difference d0 that corresponds to the time section T1 of the shift time Δt, the distance difference d0 within the smallest section can be set in the object detection device 1. Furthermore, the fluctuation widths Vpp1, Vpp of each receiving signal Sr1, Sr do not necessarily have to be strictly defined by the maximum and minimum values of the receiving signals Sr1, Sr2 corresponding to the direct wave W0, and can be defined within an appropriate range of allowable error.

Figure 13 shows an example of the received signal Sr1 when a direct wave W0 of the second frequency band Fn is received, similar to Figure 10. Figure 14 shows the relationship between the shift time Δt and the fluctuation width Vpp in the addition result of the received signal Sr1 of Figure 13, similar to Figure 11.

When the second frequency band Fn is used for the chirp wave W1, instead of narrowing the frequency band compared to the above case, as shown in FIG. 14, the section T1 of the shift time Δt in which the fluctuation width Vpp of the sum result is equal to or smaller than the fluctuation width Vpp1 before the addition is increased compared to the case of FIG. 11. This improves the degree of freedom in the arrangement of each receiver 11a, 11b, which can reduce the influence of the direct wave W0. Also, in this example, the shift time Δt in which the fluctuation width Vpp of the received signal Sr after addition is minimized is the same as the example of FIG. 11.

Figure 15 shows an example of the received wave signal Sr1 when a direct wave W0 of the third frequency band Fn is received, similar to Figure 10. Figure 16 shows the relationship between the shift time Δt and the fluctuation width Vpp in the addition result of the received wave signal Sr1 of Figure 13, similar to Figure 11.

From the viewpoint of canceling out the direct wave W0, it is desirable for the frequency band of the chirp wave W1 to have a minimum frequency equal to or greater than half the maximum frequency, such as the first and second frequency bands Fm and Fn, but the minimum frequency may be less than half the maximum frequency. For example, even when the third frequency band Fw is used, as shown in FIG. 16, a section T1 of the shift time Δt in which the fluctuation width Vpp of the addition result is equal to or less than the fluctuation width Vpp1 before addition is obtained. In this example, the shift time Δt in which the fluctuation width Vpp of the received signal Sr after addition is minimum is also the same as the example in FIG. 11.

The object detection device 1 of this embodiment can be configured by applying the optimization of the distance difference d0 described above to various frequency characteristics of the receiver 11. Figure 17 shows another example of the frequency characteristics of the receiver 11 in the object detection device 1. In this example, the frequency characteristics are illustrated when the microphone IM73A135V01 manufactured by Infineon Technologies is used as the receiver 11.

Figure 18 shows an example of the received wave signal Sr1 when a direct wave W0 of the first frequency band Fm is received in the frequency characteristics of Figure 17, similar to the example of Figure 10. Figure 19 shows the relationship between the shift time Δt and the fluctuation width Vpp in the addition result of the received wave signal Sr1 of Figure 18, similar to Figure 11.

According to the frequency characteristics in Fig. 17, as shown in Fig. 18, the swell of the received signal Sr1 has changed from the case in which the same frequency band is used in the frequency characteristics in Fig. 9 (Fig. 10). Therefore, as shown in Fig. 19, the fluctuation width Vpp according to the direct wave W0 in the received signal Sr after addition by various shift times Δt also changes from the above case (Fig. 11).

For example, in the example of Figure 19, there is one section T1 in the received signal Sr1 of one receiver 11 where the fluctuation range Vpp after addition is less than the fluctuation range Vpp1. Also, the shift time Δt at which the fluctuation range Vpp reaches the minimum peak P1 is Δt = 10 μs. Even in such a case, by using the optimization method described above to select the shift time Δt with a focus on the fluctuation range Vpp after addition, it is possible to set the distance difference d0 optimized in accordance with the frequency characteristics of the received signal Sr in the object detection device 1.

Figure 20 illustrates the received signal Sr1 when receiving a direct wave W0 of the second frequency band Fn in the frequency characteristics of Figure 17, similar to Figure 13. Figure 21 illustrates the relationship between the shift time Δt and the fluctuation width Vpp in the addition result of the received signal Sr1 of Figure 20, similar to Figure 14. In this case, the minimum peak P1 was at a shift time Δt = 8 μs.

Figure 22 illustrates an example of the received signal Sr1 when a direct wave W0 of the third frequency band Fn in the frequency characteristics of Figure 17 is received, similar to Figure 15. Figure 23 illustrates the relationship between the shift time Δt and the fluctuation width Vpp in the addition result of the received signal Sr1 of Figure 22, similar to Figure 16. In this case, the minimum peak P1 was at a shift time Δt = 13 μs.

3. Summary The object detection device 1 according to the present invention detects an object 3 by transmitting and receiving sound waves. The object detection device 1 includes a wave transmitter 10, a first wave receiver 11a, a second wave receiver 11b, a wave receiving circuit 16 which is an example of an adder, and a control unit 13. The wave transmitter 10 transmits a chirp wave W1 which is an example of a modulated wave having a predetermined frequency band to the object 3. The first wave receiver 11a receives sound waves such as an echo W2 and a direct wave W0, and generates a wave receiving signal Sr1 as a first wave receiving signal indicating the reception result. The second wave receiver 11b receives sound waves at a position farther from the wave transmitter 10 than the first wave receiver 11a, and generates a wave receiving signal Sr2 as a second wave receiving signal indicating the reception result. The adder adds the first wave receiving signal and the second wave receiving signal to generate a wave receiving signal Sr as a third wave receiving signal. The control unit 13 causes the wave transmitter 10 to transmit the modulated wave and detects the object 3 based on the third received wave signal. The first and second wave receivers 11a and 11b are arranged so that when the first and second wave receivers 11a and 11b receive the modulated wave from the wave transmitter 10, the fluctuation width Vpp of the third received wave signal corresponding to the modulated wave is equal to or smaller than the fluctuation width Vpp1 of the first received wave signal corresponding to the modulated wave.

According to the object detection device 1 described above, due to the arrangement of the first and second receivers 11a, 11b, even if the direct wave W0 is received by the first and second receivers 11a, 11b from the transmitter 10, the effect of the direct wave W0 is cancelled out in the received signal Sr (third received signal) which is the sum of the received signals Sr1, Sr2. This makes it possible to reduce the effect of the direct wave W0 received directly, not through the object 3, in the detection of the object 3 by transmitting and receiving sound waves.

In addition, in the above object detection device 1, the echo W2 from the object 3 can reach the first and second receivers 11a and 11b without any particular time difference, so the signal component corresponding to the echo W2 can be increased in the received wave signal Sr resulting from the addition. Therefore, according to the object detection device 1 of this embodiment, the signal component can be increased while reducing the noise component of the direct wave W0, thereby improving the signal-to-noise ratio in object detection.

In this embodiment, the fluctuation width Vpp of the third receiving signal according to the modulated wave repeatedly increases and decreases according to the shift time Δt corresponding to the distance difference d0 between the distance d1 from the transmitter 10 to the first receiver 11a and the distance d2 from the transmitter 10 to the second receiver 11b (FIG. 10, etc.). The fluctuation width Vpp is less than the fluctuation width Vpp1 of the first receiving signal in the shift time Δt (or the corresponding distance difference d0) of one or more sections T1. The first and second receivers 11a and 11b are arranged, for example, so that the distance difference d0 is included in the smallest section of one or more sections converted into distance from each section T1. This makes it possible to, for example, miniaturize the object detection device 1 while canceling out the effect of the direct wave W0, making it easier to configure the object detection device 1.

In this embodiment, the first and second receivers 11a and 11b have common frequency characteristics. This makes it possible to easily cancel the influence of the direct wave W0 between the first and second receivers 11a and 11b.

In this embodiment, the frequency band of the modulated wave by the transmitter 10 includes a first frequency (e.g., 80 kHz in FIG. 9) and a second frequency (e.g., 60 kHz) having a lower sensitivity than the sensitivity of the first frequency in the frequency characteristics of the receiver 11. By including a frequency with a relatively low sensitivity in the frequency band of the modulated wave, it is possible to easily achieve, for example, a wide bandwidth of the modulated wave and a reduction in the influence of the direct wave W0 at the same time.

In this embodiment, the frequency band of the modulated wave may further include a third frequency (e.g., 41 kHz) that is greater than half the first frequency and less than the first frequency. If the frequency band of the modulated wave is too wide, counter-balancing may occur in other parts when canceling out some frequency components of the direct wave W0. However, in the frequency components between the first and third frequencies, the counter-balancing can be avoided, making it easier to reduce the influence of the direct wave W0.

In this embodiment, the control unit 13 detects the object 3 by calculating a cross-correlation function based on the transmission signal Sd that transmits the modulated wave to the transmitter 10 and the third reception signal (see FIG. 5). In such an object detection method using cross-correlation processing, even if the time period during which the receiver 11 receives the sound wave is long, the object detection device 1 of this embodiment can reduce the influence of the direct wave W0.

In this embodiment, the wave transmitter 10 is composed of a thermophone that transmits modulated waves by repeatedly turning on and off heat generation. The thermophone realizes modulated waves with a wide frequency band, making it easier to detect objects.

(Embodiment 2)
In the above-mentioned embodiment 1, the object detection device 1 including the two receivers 11a and 11b has been described, but the number of receivers 11 is not limited to two and may be three or more. In embodiment 2, such a modified example will be described with reference to FIG. 24.

Figure 24 illustrates an example of the arrangement of the wave transmitter 10 and the wave receiver 11 in the object detection device 1A of embodiment 2. The object detection device 1A of this embodiment may include four wave receivers 11a, 11b, 11c, and 11d, for example, in a configuration similar to that of the object detection device 1 of embodiment 1. In this case, the wave receiving circuit 16A in the object detection device 1A includes, for example, as shown in Figure 24, an adder for one set of received wave signals from 11a and 11b, an adder for another set of received wave signals from 11c and 11d, and an adder that adds two received wave signals respectively added by the two adders.

In the object detection device 1A having the above-mentioned configuration example, for example, the distance difference that cancels out the direct wave W0 for the pair of two receivers 11a, 11b is set by the optimization method described above, and a similar distance difference is set for the remaining pair of two receivers 11c, 11d. Furthermore, the distance difference for the two pairs of receivers 11a to 11d may be optimized. By optimizing the arrangement of the multiple receivers 11a to 11d in this way, the influence of the direct wave W0 in object detection can be further reduced.

In addition, the object detection device 1A of this embodiment may include a larger number of receivers 11. For example, when 2 to the power of n receivers 11 are used, combinations of two receivers 11 may be sequentially optimized in the same manner as described above. Alternatively, in the multiple receivers 11 included in the object detection device 1A, optimization of the arrangement may be limited to the extent that the direct wave W0 is cancelled out between the two receivers 11.

As described above, the object detection device 1A of this embodiment further includes one or more third receivers 11c, 11d arranged at positions different from the first and second receivers 11a, 11b. The receiver circuit 16A as an adder adds the received signals from the first to third receivers 11a to 11d to generate a third received signal. This makes it possible to use multiple receivers 11 to increase the signal component of the echo W2 while reducing the influence of the direct wave W0, making it easier to detect objects.

Other Embodiments
In the above-described first and second embodiments, the object detection device 1 and 1A are illustrated as having a plurality of receivers 11 arranged adjacent to each other, but the object detection device 1 is not particularly limited to this. Such a modified example will be described with reference to FIG.

Figure 25 illustrates an example of the arrangement of the wave transmitter 10 and the wave receiver 11 in the object detection device 1 of this modified example. In the object detection device 1, the multiple wave receivers 11a, 11b do not have to be lined up in one direction from the wave transmitter 10. In the example of Figure 25, the first wave receiver 11a is arranged on the -X side of the wave transmitter 10, and the second wave receiver 11b is arranged on the +X side. In this way, the first and second wave receivers 11b may be arranged on either side of the wave transmitter 10. Furthermore, the directions of the receivers 11a, 11b from the wave transmitter 10 do not have to be on the same straight line, and may be set appropriately between the X and Y directions.

Even in the above-mentioned arrangement, the distance difference d0 between the first and second receivers 11a and 11b can be defined as the magnitude of the difference between the distance d1 from the transmitter 10 to the first receiver 11a and the distance d2 from the transmitter 10 to the second receiver 11b. If such a distance difference d0 is set according to the shift time Δt selected by the above-mentioned optimization method, the arrangement of each receiver 11a and 11b can be appropriately adjusted within a range that satisfies the set distance difference d0.

In addition, in each of the above embodiments, an example has been described in which the transmitter 10 and the receiver 11 are arranged on the same plane in the object detection device 1, but an arrangement other than on the same plane may also be adopted. Such a modified example will be described with reference to FIG. 26.

Figure 26 illustrates an example of the arrangement of the wave transmitter 10 and the wave receiver 11 in the object detection device 1 of this modified example. In the object detection device 1 of this modified example, in a configuration similar to that of the object detection device 1 of embodiment 1, for example, the wave transmitter 10 and the multiple wave receivers 11a, 11b are arranged with a shift in the Z direction. For example, the wave transmitter 10 is arranged on the +Z side facing the object 3, and each of the wave receivers 11a, 11b is arranged on the -Z side of the wave transmitter 10. This makes it possible to reduce the degree to which the direct wave W0 from the wave transmitter 10 is received by each of the wave receivers 11a, 11b.

In addition, in each of the above embodiments, calculations using a complex-valued cross-correlation function have been exemplified in the object detection operation by the object detection device 1, but the calculations of the object detection operation are not particularly limited to this. For example, the object detection device 1 of this embodiment may use a cross-correlation function without particularly complexifying it. For example, the control unit 13 of the object detection device 1 may calculate the distance to the object 3 by peak detection of the real part of the signal I instead of peak detection of the envelope E(t). In this case, for example, in the functional configuration of the control unit 13, the Hilbert transform unit 133 and the subsequent IFFT unit 134b can be omitted.

Reference Signs List 1, 1A Object detection device 10 Transmitter 11, 11a to 11d Receiver 13 Control unit 14 Memory unit 15 Transmitting circuit 16, 16A Receiving circuit

Claims (8)

An object detection device that detects an object by transmitting and receiving sound waves,
A transmitter that transmits a modulated wave having a predetermined frequency band to the object;
a first receiver that receives the sound waves and generates a first reception signal indicative of the reception result;
a second receiver that receives sound waves at a position farther from the transmitter than the first receiver and generates a second reception signal indicating a reception result;
an adder that adds the first received signal and the second received signal to generate a third received signal;
a control unit that causes the wave transmitter to transmit the modulated wave and detects the object based on the third wave receiving signal,
An object detection device in which the first and second receivers are arranged so that when the first and second receivers receive the modulated wave from the transmitter, the fluctuation range of the third received signal corresponding to the modulated wave is equal to or less than the fluctuation range of the first received signal corresponding to the modulated wave. The fluctuation width of the third received signal according to the modulated wave repeatedly increases and decreases according to the distance difference between the distance from the transmitter to the first receiver and the distance from the transmitter to the second receiver, and becomes equal to or smaller than the fluctuation width of the first received signal in one or more distance differences;
The object detection device according to claim 1 , wherein the first and second receivers are arranged so that the distance difference is included within a minimum section of the one or more sections. 3. The object detection device according to claim 1, wherein the first and second receivers have a common frequency characteristic. The object detection device according to claim 3 , wherein the frequency band of the modulated wave includes a first frequency and a second frequency having a lower sensitivity than the first frequency in the frequency characteristics. 5. The object detection device according to claim 4, wherein the frequency band of the modulated wave further includes a third frequency that is higher than the first frequency and lower than twice the first frequency. Further comprising one or more third receivers arranged at positions different from the first and second receivers;
3. The object detection device according to claim 1 , wherein the adder adds up the received wave signals from the first to third receivers to generate the third received wave signal. 3. The object detection device according to claim 1, wherein the control unit detects the object by calculating a cross-correlation function based on a transmission signal that causes the transmitter to transmit the modulated wave and the third receiving signal. 3. The object detection device according to claim 1, wherein the wave transmitter is a thermophone that transmits the modulated wave by repeatedly turning on and off heat generation.

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